Thermoelectric component

ABSTRACT

A thermoelectric component includes a first and a second element which, in the vicinity of a contact point, are in contact with each other, e.g., in the form of a thermal contact. Furthermore, in this connection, first element and/or second element have a ceramic material at least in one vicinity of contact point. The component may be suitable as a thermocouple for measuring temperature based on the Seebeck effect, or for use in a Peltier element as a thermoelectric heating element or cooling element based on the Peltier effect.

FIELD OF THE INVENTION

The present invention relates to a thermoelectric component, e.g., athermocouple.

BACKGROUND INFORMATION

In using thermocouples for measuring temperatures, one frequently runsinto limits of material load capacity with regard to temperature andapplication atmosphere. In using thermocouples for a temperature rangeup to 1500° C. based on PtRh/Pt elements, in extended use above 1000°C., there frequently appears a drift in the thermoelectric voltage, andsimultaneously, a considerable deterioration of the mechanicalproperties on account of creep processes. In particular, during contactwith carbon in such thermocouples, often metal carbides will form, whichchange the Seebeck coefficient and the mechanical properties of thethermocouple. In addition, PtRh/Pt thermocouples are very expensive toproduce, and therefore not usable in certain applications.

It is an object of the present invention to make available athermoelectric component that may be applied, e.g., as a thermocouple,and which may permit making a precise temperature measurement even inranges of durably high temperatures and/or transient temperature loads,as well as in oxidizing and also reducing gas atmospheres.

SUMMARY

The thermoelectric component according to the present invention mayprovide the advantage that it has a very good service life and very goodconstancy in the thermoelectric voltage that occurs, even at hightemperatures and reactive gas atmospheres, a typical service life beingconsidered to be five years. In particular, the thermoelectric componentaccording to the present invention, when used as a thermocouple, permitsmaking temperature measurements of up to 1300° C. as well as in anoxidizing and also in a reducing atmosphere, at a precision of less than±10° C. The thermoelectric component may have a short response time totemperature changes, which is typically less than one second.

Besides that, the thermoelectric component according to the presentinvention may be constructed in a small size, so that with it,microstructured thermocouples or microstructured thermoelectriccomponents may be made. A microstructured component is understood tomean a component, the element of which has typical dimensions in themicrometer range.

Because of its good temperature stability and resistance with respect toreactive gas atmospheres, in the thermoelectric component according tothe present invention, one may furthermore do without a ceramic ormetallic protective tube, so that its use as a thermocouple makespossible accurate and, at the same time, fast temperature measurement.

The thermoelectric component according to the present invention mayprovide the advantage of a long service life expectation, even inreactive gas atmospheres, at simultaneously high temperature resolutionand rapid response time. Beyond that, it can be manufacturedcost-effectively and, particularly when used as a thermocouple, it hastypical thermoelectric voltages in the mV range, which are easilymeasurable.

Thus, the thermoelectric component is not only suitable as athermocouple, but by impressing an external current upon it in aconventional manner, it may also be configured as a Peltier element, inorder to make it into, for example, a thermoelectric heating element orcooling element.

The element of the thermoelectric component may be made of a firstceramic material and a second ceramic material differing from the first,of which at least one may include additionally one or more suitablefiller materials. In this fashion, the occurring contact voltages areclearly increased, because of the Seebeck effect. Especially suitable asfiller material, e.g., for one of the elements, is a filler materialhaving at least approximately metallic conductivity, and on the otherhand, for the other element, an electrically semiconductive orinsulating filler material.

In realizing the first and/or the second element of the thermoelectriccomponent, the ceramic material of at least one element may be obtainedby pyrolysis of a polymeric precursor material or a polymeric precursormaterial provided with one or more filler materials. In this connection,by the selection of the polymeric precursor material, and by the typeand proportion of the filler materials in this precursor material, in anespecially simple manner, the thermal coefficients of expansion of theelements of the thermoelectric component may be adapted to each other.

The thermoelectric component may also be realized in that, in at leastone vicinity of the contact location, just one element is made of aceramic material, while the second element is made of a metal that maybe soldered.

The present invention is explained in greater detail with reference tothe drawings and in the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a thermoelectric component in the formof a thermocouple.

FIG. 2 is a graph of the contact voltage or the thermoelectric voltagenear the contact location of the two elements of the thermocouple as afunction of the temperature at the contact point.

DETAILED DESCRIPTION

The example embodiments further explained, first of all relate topolymeric precursor materials, provided with filler materials, which areconvertible into ceramic materials by pyrolysis. Such precursormaterials or filler materials, respectively, are described in EuropeanPublished Patent Application No. 0 412 428 or in German Published PatentApplication No. 195 38 695, which also describe that one may producemolded articles using pyrolysis, by the addition of filler materials tothe polymeric precursor materials used. In this connection, the specificresistance of the ceramic molded articles obtained may be set both bythe choice of the filler materials and by the choice of the polymericprecursor material.

As the polymeric precursor materials for the example embodiments whichare further explained, polymers may be suitable which are convertible bypyrolysis into ceramic materials based on Si—C compounds, Si—C—Ncompounds, Si—Ti—C—O compounds, Si—C—O compounds, Si—B—C—N compounds,Si—B—C—O compounds, B—C—N compounds, Si—Al—C—O compounds, Si—Al—N—C—Ocompounds or Si—C—O—N compounds.

As filler materials in these polymeric precursor materials or theceramic materials obtained after pyrolysis, respectively, fillermaterials are suitable which have at least approximately metallicconductivity such as MoSi₂, Cr₃C₂, TiC, WC, TiN, FeCr, FeCrNi, ZrN orZrC. Besides those, or alternatively to them, an electricallysemiconductive or insulating filler material such as Al₂O₃, SiC, B₄C,BN, ZrO₂, SiO₂, Si₃N₄ or graphite also come into consideration as fillermaterial.

As a low-ohm, high temperature-stable filler material havingapproximately metallic conductivity, e.g., molybdenum disilicide havinga specific electrical resistance of 2×10⁻⁵ Ωcm and a positivetemperature coefficient of the electrical resistance of 5×10⁻¹ K⁻¹ maybe suitable.

As a high-ohm insulating, high temperature stable filler material, Al₂O₃may be used, having a specific electrical resistance of more than 10¹¹Ωcm at room temperature, which is combined with a ceramic material basedon an Si—O—C compound as a conductivity carrying phase, the specificelectrical resistance of which after pyrolysis at 1400° C. about 2 Ωcm.

A first example embodiment of the present invention is explained withreference to FIG. 1. FIG. 1 illustrates a thermoelectric component inthe form of a thermocouple 5, which has a first element 10 and a secondelement 11 which are connected to each other by a contact point 12 inthe form of a thermal contact. It is further provided that thermocouple5 is interconnected with a device for measuring the contact voltage.Thermocouple 5 is used for measuring a temperature to which contactpoint 12 is exposed. This temperature typically is in the range of 0° C.to 15000° C.

FIG. 2 illustrates the plot of thermoelectric voltage which occurs inthe vicinity of contact point 12 of thermocouple 5, as a function of thetemperature to which contact point 12 is exposed. One may see from FIG.2 that the thermoelectric voltage that appears, is in the mV range, andthat, in the range of approximately 50° C. to 1000° C., it is a linearfunction of the temperature.

In the example embodiment explained, thermocouple 5 in its two elements10, 11 further includes a single pyrolysis ceramic filled, however, withtwo different filler materials, the electrical properties of which withregard to the Seebeck coefficient and the specific electrical resistanceof first element 10 and second element 11 have each been setspecifically by the type of the filler material.

The shaping of thermocouple 5 before the pyrolysis was done usingfamiliar manufacturing processes of plastic methodology, such astransfer molding, injection molding or hot pressing.

Especially important for the functioning of thermocouple 5 may be thevicinity of contact point 12 in which the two materials of first element10 and second element 11 meet each other. In this contact area, in whichthe thermoelectric voltage appears, it may be important that the twomaterials of the first and the second element 10, 11 each be ashomogeneous as possible in the vicinity of contact point 12.

During pyrolysis of the first used polymeric precursor materials, whichform elements 10, 11, in order to avoid having different shrinkages infirst element 10 or second element 11 occur, which may lead,particularly in the area of contact point 12, to cracks and therebymalfunctioning of the thermoelectric component, it is further provided,in an example embodiment, that the materials employed before thepyrolysis be adapted to one another with respect to the shrinkageappearing during pyrolysis. This adaptation may be made by the selectionof the filler materials and their proportion in each polymeric precursormaterial.

In addition to the adaptation with respect to shrinkage, it may beprovided further that the thermal coefficients of expansion of thematerial of the first element 10 and the material of the second element11 be adapted to each other, so as to minimize or avoid stresses and/orcracks in the area of contact point 12 during the operation ofthermocouple 5.

Within the framework of the explained example embodiment, theelectromotive force or the Seebeck coefficient of the materials of firstelement 10 and of second element 11 is set only by the type of fillermaterial used, whereas both elements 10, 11 are otherwise made of thesame polymeric precursor material before the pyrolysis. Thus, for one ofthe elements 10, 11 of thermocouple 5, molybdenum silicide is used ashigh temperature-resistant filler material, having approximatelymetallic conductivity. In that case, electrically semiconductive orinsulating filler materials, such as aluminum oxide or graphite may beused. In addition, however, it is also possible to make one of the twoelements 10, 11 of thermocouple 5 completely of a metal that may besoldered, such as Vacon (manufacturer: VAC Vakuumschmelze) that is, aNi—Co alloy having a low thermal coefficient of expansion. In that case,the second element of thermocouple 5 is made of the ceramic materialwhich is filled with one of the filler materials discussed.

An alternative example embodiment of thermocouple 5 provides that, asmaterials for first element 10 or second element 11 two differentpolymeric precursor materials be used, which will be present afterpyrolysis in the form of two different ceramic materials, such as aSi—Ti—C—O compound on the side of element 10 and a Si—C—O compound onthe side of the other element 11.

In this case, contact point 12 is composed in the form of a thermalcontact having a thermal voltage appearing for a thermocouple 5 fromadjacent pyrolytic ceramics of different composition, e.g., havingdifferent filler materials.

Besides the kind of filler material, alternatively or additionally tothe preceding example embodiments, the proportion of the filler materialin the polymeric precursor material(s) used may also be varied, so as toset the thermoelectric and the mechanical properties in this fashion,e.g., the Seebeck coefficient in contact area 12 of the thermocouple 5so obtained.

The entire filler material content is between 10% by volume and 50% byvolume, in relation to the volume of the initial blank present beforethe pyrolysis, including the polymeric precursor materials discussed.

The thermal voltage or the Seebeck coefficient, appearing in the area ofcontact point 12, may also be set, within certain limits, by the methodparameters during pyrolysis, in all the preceding example embodiments.

Subsequently, an example embodiment for producing a thermocouple 5illustrated in FIG. 1 will now be explained in greater detail.Alternative example embodiments may be realized in view of EuropeanPublished Patent Application No. 0 412 028 or German Published PatentApplication No. 195 38 695, by varying the type and the amount of thefiller materials used, or of the polymeric precursor compounds used.

To begin with, 53.1 g pulverulent condensation-cross-linkedpolymethylsiloxane and 46.9 g Al₂O₃ powder are placed in a grinding millper 1000 g of steel grinding balls. This corresponds to a filling ratioof 20% by volume of Al₂O₃ with respect to the polymer filler materialmixture. After a grinding time of 5 minutes, the powder mixture obtainedis separated from the steel balls and screened using a 150 μm sieve.Subsequently, the screened powder mixture is filled into a mold and iscold-molded at a pressure of 150 MPa. The first powder mixture is thusused as a first polymeric precursor material furnished with a firstfiller material, from which subsequently first element 10 ofthermocouple 5 will be formed.

For second element 11 of thermocouple 5, likewise, to begin with, 35.3 gpulverulent, condensation-cross-linked polymethylsiloxane and 64.7 gmolybdenum silicide are used per 1000 g of steel grinding balls. Thiscorresponds to a filling ratio of 25% by volume of molybdenum silicidewith respect to the polymer filler material mixture. After grinding andscreening, which is performed as described above, the powder mixture isthen filled, as second polymeric precursor material furnished with asecond filler material, into the mold, in which there is already thematerial for first element 10. After a cold-mold procedure at 150 MPa,the material junction thus obtained is next age-hardened for 30 minutesat a pressure of 10 MPa and a temperature of 170° C.

Subsequently, U-shaped molded articles as in FIG. 1 are separated out(of the mold), which are then pyrolyzed according to the followingtemperature program under a flowing argon atmosphere having an argonflow of 5 l/min. Thermocouple 5 obtained from this temperature programhas a thermal voltage which is in the thermal voltage range of knownthermocouples based on PtPh/Pt. The temperature dependence of theoccurring thermal voltage of thermocouple 5 that is obtained is shown inFIG. 2.

Heating Rate/ Final Temperature Retention Time Cooling Rate (° C./h) (°C.) (hours) 300 300 0 20 900 0 80 1400 1 150 20 —

Thermocouple 5 as in FIG. 1 has typical dimensions of width of elements10, 11 of 10 μm to 1 cm and a thickness of elements 10, 11 of 1 μm to 1cm. Furthermore, the typical length of elements 10, 11 is in the rangeof 1 cm and more. The distance apart of the first and second element 10,11 in the region of thermocouple 5, in which these two elements 10, 11extend parallel to each other, is between 50 μm and 5 cm. Thermocouple 5may thus be made even as a microstructured thermocouple having typicaldimensions in the micrometer range. Besides, it is clear that, insteadof a thermocouple 5, a thermoelectric component in the form of a Peltierelement may also be made in the manner explained above. Then too, morethan one contact point 12 may be provided, which are made of relevantmaterial combinations for elements 10, 11 that define these contactpoints 12.

The geometry of thermocouple 5 may not be limited to the U-shapeexplained with reference to FIG. 1, i.e., there are other geometriestoo, and other dimension of the thermoelectric component which may berealized, according to the response time desired.

1. A thermoelectric component, comprising: a first element; and a secondelement; wherein the first element and the second element are in contactwith each other in an area of at least one contact point; and wherein atleast in one vicinity of the contact point, at least one of the firstelement and the second element includes a ceramic material, wherein atleast in one vicinity of the contact point, the ceramic materialincludes a filler of one of FeCr and FeCrNi.
 2. The thermoelectriccomponent according to claim 1, wherein the thermoelectric componentincludes a thermocouple.
 3. The thermoelectric component according toclaim 1, wherein at least in one vicinity of the contact point, thefirst element includes a first ceramic material and the second elementincludes a solderable metal.
 4. The thermoelectric component accordingto claim 1, wherein the filler material includes at least one of afiller material having an at least approximately metallic conductivity,an electrically semiconductive filler material and an insulating fillermaterial.
 5. The thermoelectric component according to claim 1, whereina material of the first element and a material of the second elementhave an at least approximately equal thermal coefficient of expansion atleast in the vicinity of the contact point.
 6. The thermoelectriccomponent according to claim 1, wherein in at least one vicinity of thecontact point, a material of the first element and a material of thesecond element are configured so that at the contact point one of acontact voltage occurs in accordance with a Seebeck effect and atemperature change occurs in response to an impressed external electriccurrent in accordance with a Peltier effect.
 7. The thermoelectriccomponent according to claim 6, wherein the first element and the secondelement electrically interconnect with one of a device configured tomeasure the contact voltage and a device configured to impress anexternal electric current flowing through the contact point.
 8. Thethermoelectric component according to claim 1, wherein at least in onevicinity of the contact point, the first element includes a firstceramic material and the second element includes a second ceramicmaterial different from the first ceramic material.
 9. Thethermoelectric component according to claim 8, wherein at least one ofthe first ceramic material and the second ceramic material includesobtained by pyrolysis of one of a polymeric precursor material and apolymeric precursor material that includes at least one filler material.10. The thermoelectric component according to claim 8, wherein at leastone of the first ceramic material and the second ceramic materialincludes a ceramic material based on one of Si—C compounds, Si—C—Ncompounds, Si—Ti—C—O compounds, Si—C—O compounds, Si—B—C—N compounds,Si—B—C—O compounds, B—C—N compounds, Si—Al—C—O compounds, Si—Al—N—C—Ocompounds and Si—C—O—N compounds.
 11. The thermoelectric componentaccording to claim 8, wherein the first ceramic material is obtained bypyrolysis of one of a first polymeric precursor material and a firstpolymeric precursor material that includes a first filler material andthe second ceramic material is obtained by pyrolysis of one of a secondpolymeric precursor material and a second polymeric precursor materialthat includes a second filler material.
 12. The thermoelectric componentaccording to claim 11, wherein the first polymeric precursor materialand the second polymeric precursor material are configured so that, inresponse to pyrolysis of the precursor materials, an at leastapproximately equal shrinkage occurs at least in the vicinity of thecontact point.
 13. A method, comprising the steps of: providing athermoelectric component, the thermoelectric component including a firstelement and a second element, the first element and the second elementarranged in contact with each other in an area of at least one contactpoint, at least in one vicinity of the contact point, at least one ofthe first element and the second element including a ceramic material,wherein the ceramic material includes a filler of one of FeCr andFeCrNi; and arranging the thermoelectric component in one of athermocouple configured to one of measure temperature and a Peltierelement as one of a thermoelectric heating element and a coolingelement.